Dativ vs Akkusativ

Hallo!

So I thought that maybe we have people here who can help me with this:

I'm having a lot of trouble understanding when to use Dativ vs Akkusativ in German. I understand that specific prepositions require specific cases, but in general I often find myself applying the wrong case in a sentence.

In some sentences it is quite clear:

Ich habe den Stift gekauft. (Pen being the direct object that is being bought)

But there are cases like this:

Ich schlafe in meinem auto in meiner pause (I know the order is wrong, but the cases seem to stay as they are even if you change the order)

Here I would have thought that the car is the direct object. I struggle with this a lot and often apply akkusativ case wrongly.

I would appreciate if someone could help me with understanding this better. For example: Why is the car not the direct object in the above example?